Externalize Explosion Graphics Config

Hi SupSuper,

I've done some work on a change to allow different sprites to be used for explosion graphics.  Forum thread here.

http://openxcom.org/forum/index.php?topic=2927.0

Here is a loose summary of the changes:

Fixes the following bugs:
- ExplosionBState.cpp: big explosions spawn sprites in circle around
centre, not square
- Map.cpp: offset for explosion sprites from X1.PCK now correctly offset
by -64,-32 not -64,-64

This means that the layout of explosion sprites should appear more
circular (most visible on large explosions) and be correctly centred
(most visible on small explosions).

Defines the following config:

- item config contains a new explosion entry (or defaults to
STR_DEFAULT)
- new explosion entries contain config that detail animation for
different explosions

Adds following new config:

- STR_DEFAULT explosion entry for X1.PCK in Xcom1Ruleset.rul
- Improved_Explosions.rul has rules for new explosion graphics

Adds new sprites for Improved_Explosions.rul

------------

Please, let me know what changes I'd need to make to get this good enough for inclusion!  Even if you don't want the change, please consider adopting the bug fixes in as off-center explosions really interfere with my OCD :)

Since I've not meddled with Github or your project before, do feel free to set me straight if I have the etiquette on any of this wrong.